Dynamic mode decomposition of forced spatially developed transitional jets

In this paper, the dynamic mode decomposition (DMD) is employed to study the database of a swirling jet simulated by large eddy simulation (LES), and the results are compared with its non-swirling counterpart in detail, in purpose of understanding swirl-induced changes of flow dynamics at transitional stage. The two jets have identical Mach number (Ma=0.9) and Reynolds number (Re=9×104), and eigenmode forcing is applied at the inflow. For both jets, the neutrally/least stable dynamic modes are found illustrated by DMD-spectrum. The prevalent temporal frequencies from a global perspective are obtained by DMD, reasonably agree with that of LES. The dynamic modes capture the physical processes like vortex roll-up and merging accurately, including spatial structures and corresponding locations. It is found that the dominant mode is varied, more pertinent frequency contents are found, nonlinear growth rates are enhanced and nonlinear interactions are strengthened with the addition of swirl. Moreover, proper orthogonal decomposition (POD) presents some similar spatial structures to that of DMD, but differences between DMD and POD are also found in analyzing such transitional flows.